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Ground Water Quality Characterization Program

Installing a ground water probe to collect samples

As the designated state water quality management agency, Ohio EPA is responsible for defining and reporting on ambient ground water quality conditions, assessing ground water quality problems and recommending strategies for preventing contamination. Good information on past and present ambient ground water quality, the extent of ground water contamination problems and the relationship of land-use practices to ground water quality is critical to developing effective strategies to prevent ground water contamination. To provide this water quality data, the Ground Water Quality Characterization Program collects and reviews water quality data.

In Ohio, nearly half of all cities, villages, schools, businesses and industries depend on ground water for drinking, processing and irrigation. This amounts to more than 1 BILLION GALLONS PER DAY! Ground water is a shared resource. Do your part to protect it.

Do you have your own well? Visit wellowner.org or the Ohio Department of Health's Private Water Systems Program webpage.

Ambient Ground Water Monitoring

Ohio's Ambient Ground Water Quality Monitoring Program

Taking field parameters at a wellsite

Ohio EPA Division of Drinking and Ground Waters maintains the Ambient Ground Water Monitoring Program as part of an effort to characterize general water quality conditions in Ohio. This program was established in 1967 to measure seasonal and annual water quality changes in the state's major aquifers. The network initially consisted of 12 large production wells, and was expanded to 60 wells in 1972. In 1986, the network was further expanded to around 90 wells. A large number of public water supply wells were added to the network in the late 1980s and early 1990s to provide better representation of the major aquifers in Ohio.

The program currently includes over 200 wells (stations). Of the total stations, roughly 85 percent are public water systems and 15 percent are industrial or commercial enterprises or residential. Raw water is analyzed for a suite of inorganic parameters every six, 18 or 36 months depending on the total number of samples that have been collected and the stability of the geochemistry of major elements at the site. Samples are also analyzed for volatile organic compounds once every 18 or 36 months. Some ambient sites have historical semi-volatile organic compounds and pesticide data.

Locational and lithologic information have been compiled for all of the more than 200 ambient wells for effective geochemical and Geographical Information System (GIS) analysis. Two-thirds of the wells in the ambient network are developed in unconsolidated deposits and the remaining produce from bedrock aquifers systems.

A central goal of the Ambient Ground Water Quality Monitoring Program is to provide reliable ground water quality data to enhance water resource planning and protection on a state-wide basis. This is consistent with the Division of Drinking and Ground Waters' mission to protect human health and the environment by characterizing and protecting ground water quality and ensuring that Ohio's public water systems provide adequate supplies of safe drinking water.

Ambient Ground Water Quality Monitoring Locations and Water Quality Data

An interactive map of the ambient monitoring well locations has been developed. This application allows the user to zoom into an area in Ohio and click on a monitoring location. Information such as water quality summary reports and time series analyses for each monitoring location can be obtained from this site.

Water Quality Data Distribution

General Ground Water Quality in Ohio

Piper diagram depicting the ground water quality of Ohio

Several factors influence ground water quality in Ohio, including the composition of the soil and vadose zone, the composition of the aquifer material and the residence time of the ground water. Although the vadose zone material is variable and influences the quality of water infiltrating to the major aquifers, the composition and solubility of the aquifer material exerts the greatest control on the local ground water quality. The more soluble the aquifer material, the greater the influence it has on water quality. For instance, carbonate rocks are more soluble than sandstones and, consequently, the calcium concentration in limestone aquifers is greater than in sandstone aquifers. The residence time of the ground water is also a major factor, with longer residence time leading to higher total dissolved and greater mineralization. 

These differences are illustrated in the piper diagram above, which provides a summary of cation data (left triangle), anion data (right triangle) and a composite diamond (center) to visually distinguish waters of different chemical composition and origin. The small symbols are the individual mean concentrations for each ambient station, and the squares are the average concentrations for the major aquifer type.

GREEN POINTS: The carbonate ground waters in the piper diagram trend toward a more magnesium- and sulfate-rich composition, reflecting the dissolution of dolomite and gypsum minerals within the carbonate stratigraphic section. Carbonate ground waters also demonstrate higher concentrations of calcium than water from sandstones and sand and gravel aquifers.

RED POINTS: The sandstone water chemistry reveals a higher mean sodium, potassium and chloride contents than the other two systems, indicating a probable natural source for these ions from deeper formation waters. Most of the higher sodium concentrations are associated with deeper wells supporting the influence of deep formation waters.

BLUE POINTS: The overlap in water chemistry between the carbonate aquifer and the sand and gravel aquifer is due to the fact that much of the aquifer material in the unconsolidated sand and gravel units is carbonate bedrock that was eroded and transported by the glaciers and deposited within the buried valley aquifers. The concentrations of many constituents in the ground water from the sand and gravel aquifers is intermediate between the carbonate and sandstone due to the mixing of local bedrock into the buried valley sand and gravel deposits.


Fluoride Distribution in Ohio's Ground Water

State-wide fluoride distribution     Fluoride probability map

Fluoride is the naturally occurring stable form of the gaseous element fluorine (F). Fluoride is among the top 15 most abundant components of the Earth's crust and is naturally found in very small amounts in most aquifers. An aquifer is an underground unit of saturated earth materials that can provide usable quantities of ground water to a well.

The maps on this page show that the limestone aquifers in western Ohio have the highest fluoride concentrations in Ohio. Limestone aquifers contain the most fluoride-bearing minerals and dissolution of these minerals releases fluoride into ground water in the limestone aquifers. The sand and gravel aquifers found throughout the state may contain some fluoride-bearing carbonate material from local erosion. Thus, the fluoride concentrations in some sand and gravel aquifers falls between the higher concentrations found in the limestone aquifers and the lower concentrations in the sandstone aquifers.


Manganese Distribution in Ohio's Ground Water

Manganese distribution in Ohio's ground waters

Manganese solubility is controlled redox reactions. If ground water is oxidized, manganese is not soluble and, consequently, manganese concentrations are low. In reduced conditions, manganese is soluble and the concentrations of dissolved manganese increase. The figure illustrates that redox conditions in the sandstones and particularly buried valley aquifers are the best for elevated manganese. This is confirmed by the fact that many public water systems using buried valley aquifers need to remove manganese.


Sodium Distribution in Ohio's Ground Waters

Sodium distribution in Ohio's ground water

The map of sodium concentrations in Ohio does not exhibit associations of sodium with a major aquifer. Plotting box plots of sodium concentrations against major aquifer types confirms the lack of strong associations, but the higher concentrations of sodium are associated with sandstone aquifers, probably associated with deep formation waters.


Sulfate Distribution in Ohio's Ground Waters

Sulfate Distribution in Ohio's Ground Waters

The map of sulfate concentrations exhibits a strong correlation between elevated sulfate and carbonate bedrock in northwest Ohio. Much of the sulfate is derived from the dissolution of gypsum (CaSO4  2H2O), which is present in the Devonian Salina Group.

Total Dissolved Solids

Total Dissolved Solids Distribution in Ohio's Ground Waters

TDS distribution in Ohio's ground water

Total dissolved solids (TDS) are variable across the state, but limestone bedrock aquifers consistently demonstrate elevated TDS. The high solubility of limestone and the presence of gypsum interbedded with limestone contributes to the elevated TDS in northwest Ohio.

Ohio's Aquifers

Major Aquifers in Ohio

TDS distribution in Ohio's ground water

Ohio's aquifers can be divided into three major types: sand and gravel; sandstone; and carbonate.

The sand and gravel valley aquifers are distributed through the state. The valleys these sands fill are cut into sandstone and shale in eastern Ohio and into carbonate aquifers in western Ohio. The sandstone and carbonate aquifers generally provide sufficient production for water wells except where dominated by shale, as in southeast Ohio.

Sole Source Aquifers in Ohio

U.S. EPA defines a Sole Source Aquifer (SSA) as an aquifer that supplies at least 50 percent of the drinking water consumed in the area overlying the aquifer. These areas may have no alternative drinking water source(s) that could physically, legally and economically supply all those who depend on the aquifer for drinking water.

The Sole Source Aquifer designation protects an area's ground water resource by requiring U.S. EPA to review certain proposed projects within the designated area. All proposed projects receiving federal funds are subject to review to ensure that they do not endanger the water source.

Greater Miami SSA

Allen County SSA

Catawba Island SSA

Pleasant City SSA

For more information on Sole Source Aquifers, visit U.S. EPA's website.

Access Data

Accessing Water Quality Data

Raw Data

Downloads of the complete Ambient Ground Water database for both inorganic and organic ground water data may be accessed through the links below. Two data formats for each dataset are provided: .xlsx (Microsoft Open Office XML) and .csv (comma-separated values). The .xlsx format was chosen because it lacks the worksheet size limits of earlier Excel versions. The .csv is a standard text format that can be imported to wide variety of software products. Readers needing alternative formats are encouraged to direct email requests to Katharine.Schleich@epa.ohio.gov

Summarized Data


Ohio's Report Series on Ground Water Quality 


Reduction-Oxidation (Redox) Processes:

Major Aquifers in Ohio:


Ohio's Integrated Water Quality Monitoring and Assessment Report

The Integrated Water Quality Monitoring and Assessment Report indicates the general conditions of Ohio's waters and identifies waters that are not meeting water quality goals. The links below point to the specific chapter in these biennial reports that discusses Ground Water Quality in Ohio. Reports prior to 2010 are available upon request. To view the entire report, visit the Integrated Report web page.


Fact Sheets

Special Investigations